Water Budgets and Potential Effects of Land- and Water-Use Changes for Carson Valley, Douglas County, Nevada, and Alpine County, California

To address concerns over continued growth in Carson Valley, the U.S. Geological Survey, in cooperation with Douglas County, Nevada, began a study in February 2003 to update estimates of water-budget components in Carson Valley. Estimates of water-budget components were updated using annual evapotranspiration (ET) rates, rates of streamflow loss to infiltration and gain from ground-water seepage, and rates of recharge from precipitation determined from data collected in 2003 and 2004 for the study and reported in the literature. Overall water budgets were developed for the area of basin-fill deposits in Carson Valley for water years 1941-70 and 1990-2005. Water years 1941-70 represent conditions prior to increased population growth and ground-water pumping, and the importation of effluent. A ground-water budget was developed for the same area for water years 1990-2005.

Estimates of total inflow in the overall water budget ranged from 432,000 to 450,000 acre-feet per year (acre-ft/yr) for water years 1941-70 and from 430,000 to 448,000 for water years 1990-2005. Estimates of total inflow for both periods were fairly similar because variations in streamflow and precipitation were offset by increases in imported effluent. Components of inflow included precipitation on basin-fill deposits of 38,000 acre-ft/yr for both periods, streamflow of the Carson River and tributaries to the valley floor of 372,000 acre-ft/yr for water years 1941-70 and 360,000 acre-ft/yr for water years 1990-2005, ground-water inflow ranging from 22,000 to 40,000 acre-ft/yr for both periods, and imported effluent of 9,800 acre-ft/yr for water years 1990-2005 with none imported for water years 1941-70. Estimates of ground-water inflow from the California portion of Carson Valley averaged about 6,000 acre-ft/yr and ranged from 4,000 to 8,000 acre-ft/yr. These estimates compared well with a previous estimate of ground-water inflow across the State line.

Estimates of total outflow in the overall water budget were 446,000 acre-ft/yr for water years 1941-70, and 439,000 to 442,000 acre-ft/yr for water years 1990-2005. Variations in ET and outflow of the Carson River were offset by an increase in net ground-water pumping for water years 1990-2005. Components of outflow include ET of 151,000 acre-ft/yr for water years 1941-70 and 146,000 acre-ft/yr for water years 1990-2005, streamflow of the Carson River of 293,000 acre-ft/yr for water years 1941-70 and 278,000 acre-ft/yr for water years 1990-2005, and net ground-water pumping of 2,000 acre-ft/yr for water years 1941-70, and 15,000 to 18,000 acre-ft/yr for water years 1990-2005. The decreased average flows for water years 1990-2005 compared to water years 1940-71 were likely the result of dry conditions from 1987 to 1990. The large volumes of inflow and outflow of the Carson River dominate the overall water budget.

Estimates of ground-water recharge for water years 1990-2005 ranged from 35,000 to 56,000 acre-ft/yr, and total sources of ground-water discharge ranged from 41,000 to 44,000 acre-ft/yr. Components of ground-water recharge included ground-water inflow from the Carson Range and Pine Nut Mountains (22,000 to 40,000 acre-ft/yr), ground-water recharge from streamflow (a minimum value of 10,000 acre-ft/yr), and secondary recharge of pumped ground water that returns to the water table (3,000 to 6,000 acre-ft/yr). Components of total ground-water discharge included ground-water ET from native phreatophytes, riparian vegetation, and non-irrigated pasture grasses (11,000 acre-ft/yr); ground-water discharge to streamflow of the Carson River (15,000 acre-ft/yr), and net ground-water pumping (15,000 to 18,000 acre-ft/yr).

Changes in land use between water years 1941-70 and 1990-2005 have decreased ET by about 5,000 acre-ft/yr. Increased application of effluent for irrigation between those years has decreased the use of surface water and ground water for irrigation by about 9,500 acre-ft/yr. The total decrease, about 15,000 acre-ft/yr, was approximately equal to the net ground-water pumping of 15,000 to 18,000 acre-ft/yr. The decrease in ET and in the use of streamflow and ground water for irrigation would tend to increase outflow of the Carson River from Carson Valley, offsetting the decrease in outflow caused by ground-water pumping without changes in land use predicted by previous studies of water budgets for Carson Valley.